Distributed coherent communications are Radio Frequency (RF) communications where coherent transmissions are made from a transmit antenna array, and/or RF transmissions are received by a synchronized receive antenna array. Each of the antenna arrays may be formed by an array of nodes, with each of the nodes having one or more of the antennas. Some or all of the nodes of either transmit (Tx) node array or receive (Rx) node array may be ad hoc nodes (as is described below). The “coherent” property of the coherent communications refers to synchronization of the nodes, so that (1) each of the nodes in the transmit array can transmit synchronously, and/or (2) each of the nodes of the receive array are synchronized and the received signals may be combined using a common time reference, with resulting transmit and/or receive array gain. Distributed coherent communications may offer significant link budget gains and increased performance over those available with single-antenna-to-single-antenna communications. Compared to the use of single antenna transceivers, the use of multiple antennas in wireless networks may offer the promise of increased data rates, reach distance, battery life, anti jam capabilities, spectral reuse, reduced latency, and/or other benefits. Distributed coherence can be leveraged into transmit beamforming (with, e.g., N2-fold increase in power for N transmit antennas), receive beamforming.
Synchronization of the different nodes that are ad hoc nodes (as this is explained below, in the related applications, and in other commonly-assigned applications incorporated by reference below) is not always a trivial manner. This may be especially problematic when the one or more channels between a Master node of the array (to which other nodes of the array are synchronized) experience substantial multipath or are Non-Line-of-Sight (NLoS) channels. Additionally, in the initial alignment process, the clocks of the Slave nodes (nodes of the array other than the Master node) may not be time-synchronized to the Master node, and hence the Master node's calculation of the round-trip delay times may be in error to an arbitrary degree, compared to what the Master node could have calculated with proper time synchronization.
Synchronization (alignment of phases, frequencies, and time bases of the nodes of an array) is necessary for the array to operate as a phased array or a Time-Reversal mirror that can focus transmissions on one or more targets, while possibly nulling (reducing) the energy transmitted to other points, such as at hostile receivers. Therefore, there is a need in the art for techniques for improving radio frequency communications, and in particular for techniques for synchronizing arrays of ad hoc nodes. In particular, there is a need for improved techniques for synchronizing ad hoc nodes in both static and dynamically-changing environments, and where the nodes are located Line-of-Sight (LoS) and/or Non-Line-of-Sight of other nodes of the array. Additionally, there is a need in the art for improved array synchronization techniques for arrays that transmit targeted destructive electromagnetic pulses (“EMPs”).